Oils obtained from the usual oil and fat production processes by compressing oil-bearing materials or by extracting oil from the materials and removing the extraction solvent (hereinafter, referred to as "crude oil") contain impurities such as polar lipids mainly composed of phospholipids, as well as fatty acids, pigments, odor components and the like. Thus, it is necessary to remove these impurities by a refining process. The refining process requires a degumming step and an alkali refining step. In the degumming step, hydration of phospholipids and the like is effected by adding hot water to the crude oil and gum materials are removed by centrifugation. In the alkali refining step free fatty acids in the degummed oil are neutralized with caustic soda and removed by centrifugation.
Thereafter, refining of oil and fat is completed via a bleaching step in which chlorophyll and the like pigments are removed by allowing them to be adsorbed by activated clay, activated carbon or the like and a deodorization step in which odor components are removed by vacuum distillation. In the case of the production of salad oil, a dewaxing step is optionally employed in order to crystallize and remove solid fats, waxes and the like which are apt to be solidified.
However, in the alkali refining step in which free fatty acids are neutralized with caustic soda and then removed by centrifugation, residual phospholipids are also removed, but the step generates so-called "soap stocks" which contain a large quantity of accompanying oil. Though a portion of the soap stocks is used as production material for fatty acids, they are treated mostly as industrial waste.
In addition, in the subsequent neutralization step, the processed oil is washed with hot water in order to remove soap dissolved in the oil, thus generating a large quantity of oil-containing alkaline waste water which must also be treated.
These alkali refining and neutralization steps cause a great loss in the oil and fat yield.
Thus, since the conventional oil and fat refining process requires complex and time-consuming steps, great concern has been directed toward the development of a refining process which can be operated more efficiently by simplification and the like.
With regard to the omission of the alkali refining step which generates waste materials and reduces oil yield, a so-called steam refining process in which free fatty acids are removed by vacuum steam distillation in the deodorization step (JP-B-53-38281 for instance), a process in which degummed oil is treated with an enzyme having phospholipase A activity (JP-A-2-153997), a process in which a phosphatase is used (EP-A 0,070,269) and a process in which phospholipases A.sub.1, A.sub.2 and B are used (EP-A 0,513,709) have been proposed. (The term "JP-A" as used herein means an "unexamined published Japanese patent application", and the term "JP-B" means an "examined Japanese patent publication".)
However, the process of JP-B-53-38281 is limited to the refining of low phospholipid oil and fat derived from palm oil and the like materials, and it entails production of oil and fat containing a large quantity of remaining phospholipids when applied to a starting material derived from generally used oil seed such as soybean, rapeseed or the like. Such a product cannot be used commercially because of considerable coloring and odor generated by heating.
On the other hand, the processes of JP-A-2-153997, EP-A-0,513,709 and EP-A-0,070,269 require either a prolonged period of time for reaction with the oil or a large amount of enzyme.